1. Field of the Invention
This invention relates to a cutter head or cutter body for a cutting assembly which is used in gear cutting operations.
2. Background Art
Spiral bevel, zerol and hypoid gears are well known and widely used throughout the automotive industry in a variety of drivetrain applications. Conventional methods for making each of these gears include face milling and face hobbing, with the two methods producing different gear tooth geometry which must be taken into consideration during the gear design. Gears produced by face milling typically have a tapered tooth depth, with the heel end of the tooth being deeper than the toe end of the tooth but they may also have uniform depth teeth. In contrast, gears produced by face hobbing always exhibit a uniform tooth depth. Face milling is an intermittent indexing process which cuts one tooth at a time and then indexes to the next slot so as to cut the adjacent tooth. This process is repeated until all of the teeth have been cut. Face milling is referred to as a two axis system since the work, or gear, is required to rotate in timed relationship with a cradle mechanism used to mount a cutter head assembly. In contrast, face hobbing is a continuous indexing process whereby all of the gear teeth are cut simultaneously. Face hobbing is considered a three axis process since the rotation of the gear, cradle, and cutter head assembly are all performed in timed relationship with one another. Face milling is an older process, with face hobbing becoming more important with the advent of computer numerical controlled machines which allows the user to produce either geometry. Customer requirements, engineering and production requirements are considered when selecting either face milling or face hobbing to produce a gear.
Each of the cutting methods utilizes a plurality of cutting blades which are mounted into slots, being typically four-sided and formed in a face of a cutter head. The cutter head, or cutter body, is a two-piece construction comprising a first, disk-like member and a second, backing ring member which is concentric with the disk. In a known cutter head, portions of each of the blade-receiving slots are formed in both the disk and the backing ring. The two members are sized such that they mate together in an interference fit and known cooling and heating techniques are utilized to assemble the parts. For instance, the disk may be cooled, so as to shrink or reduce its outer diameter, and the concentric backing ring member may be heated so as to increase its inner diameter. After assembly, the two parts may be welded or bolted together. This method of assembling the concentric members of the cutter head is known to cause thermal distortion in the slots which receive the individual cutting blades. This method of assembly also results in internal weaknesses, particularly at the interface between the first disk-like member and the second backing ring member. When a disk-like body member and a ring member are welded together, for example, thermal stresses are set up in the region of the interface, resulting in weaknesses in this region. These weaknesses in the cutter body or cutter head allow the cutter head to be easily damaged and may lead to eventual failure. Upon damage or failure of the cutter head, attempts to repair the cutter head prove to be difficult and expensive, to the extent that such procedures are prohibitive and generally not employed. The damage or failure of the cutter head may also simply not be repairable. The two-piece configuration may further result in errors in positioning of the components which can translate into errors in cutting operations using the cutting head. In spite of the known disadvantages of a two-piece cutter head, the two-piece cutter head continues to be universally used and is the standard in the industry.
This invention according to one aspect provides a one-piece cutter body for a cutting assembly used in gear cutting operations, wherein the body is formed from a single piece of metal, and wherein further the body comprises opposite first and second faces and a sidewall. The invention further comprises a plurality of circumferentially spaced slots formed in one of the faces which extend through the body to the opposite face, the slots being adapted to receive cutting tools in positions projecting from at least one face of the body, with the slots being formed by an essentially heatless process. The preferred process for forming the slots is electron-discharge machining (EDM). The EDM process preferably utilizes computer numerical control (CNC) methods to attain highly accurate slot configurations within the one-piece body.
This invention according to a further aspect provides a method for forming a one-piece cutter body for gear cutting operations. This method comprises providing as a work piece a one-piece metal blank having opposite parallel first and second surfaces corresponding respectively to first and second faces of a desired finished cutter body, and forming in the metal blank a plurality of circumferentially spaced slots. Each slot is formed to extend from the first face to the second face, and wherein forming of the slots comprises electroerosively machining the blank until a desired slot configuration is obtained. The slots are preferably formed by electron discharge machining (EDM), utilizing computer numerical control (CNC) methods so that a highly precise slot configuration is obtained.